U.S. Pat. No. 4,707,055 issued Nov. 17, 1987 to Stark and U.S. Pat. application Ser. No. 84,426 filed Apr. 4, 1986 by Narayan, Roddy, Stark and Thompson (which are incorporated herein by reference) describe scanning apparatus for providing a substantially straight line scan of a laser spot having a selected shape. The scanning apparatus shown schematically in FIG. 2 includes a laser beam light source 10, such as a laser diode. The beam 12 from the laser diode 10 is collimated by collimating optics 14, and may be preshaped to a desired cross section by a beam stop 15 such as that shown in U.S. Ser. No. 211,938 filed by the present inventor on June 21, 1988 (which is incorporated herein by reference). The beam stop 15 is employed to preshape the beam due to the wide variability in the shapes of beams produced by individual laser diodes in a batch. As shown in FIG. 3, individual diodes in a batch of laser diodes produce beams A, B, and C having a variety of cross sectional shapes. The beam stop 15 is configured to produce a beam having a desired shape E that is within the range of shapes of all beams produced by the diodes in a batch. The beam 12, as shaped by beam stop 15 is incident on a stationary diffraction grating 16, which directs the shaped beam 12' to a rotating holographic beam scanner disc 18, referred to in the art as a hologon scanner. The hologon scanner disc 18 comprises a plurality of holographically produced diffraction grating facets 20 as shown in FIG. 4. As the beam scanner disc is rotated by a high speed motor 22, the diffraction grating facets 20 cause the beam to scan in a direction perpendicular to the plane of the drawing in FIG. 2. The scanned beam 12" then passes through a pair of beam shaping prisms 24 and 26 which serve as beam expanders for expanding the cross section of the beam in the cross scan direction. The expanded beam 12" then passes through an f-.theta. lens 28 which focuses the scanning beam onto a target such as a rotating drum 30. The beam is scanned by hologon 18 in a direction parallel to the axis 32 of the rotating drum 30 to provide a line scan of a photosensitive element 34 attached to the drum 30. A page scan of the element 34 is provided by rotation of drum 30 about axis 32. Means (not shown) are provided for modulating the output of the laser diode 10 in accordance with information contained in a stream of electronic signals. As such means are well known in the art, no further description is provided herein. For optimum exposure of the photosensitive element 34, it is desirable that the spot produced by the laser beam be narrower in the line scan direction than in the page scan direction. For large format scanning apparatus, it is also desirable that the useful duty cycle of the hologon be as large as possible, and for a high resolution scanner (i.e. a small spot at the photosensitive element 34) it is desirable that the beam at the hologon be large. These requirements dictate that the spot 12 at the hologon be narrower in the page scan direction, as shown in FIG. 4. The prisms 24 and 26 are employed to generate the desired beam shape at the photosensitive element 34.
In the scanning apparatus described above, there is a continuing effort to reduce the manufacturing cost of the apparatus by reducing the number of parts while maintaining the high resolution capabilities of the scanning apparatus.